1. Field of the Invention
The present invention is directed to the field of electroluminescent (EL) devices, and in particular to organic materials for organic light emitting devices (OLEDs).
2. Description of the Related Art
OLEDs are typically comprised of at least a layer of organic luminescent material sandwiched between an anode, typically comprised of a transparent conductor, such as indium-tin oxide and a cathode, typically a low work-function metal, such as magnesium, calcium, aluminum, or the alloys thereof, with other metals. When a bias is applied between the electrodes, positive charges (holes) and negative charges (electrons) are respectively injected from the anode and cathode into the luminescent layer. The holes and the electrons form excitons in the organic layer to emit light.
OLEDs having multiple organic layers are also known. A multilayer OLED may comprise one or more organic hole transport layers adjacent the anode, and one or more organic layers adjacent the cathode which function as both an emissive layer and an electron transport layer. In other structures, a hole transport layer, an emissive layer and an electron transport layer are positioned, in that order, between the anode and the cathode.
Organic materials for EL devices are attractive due to their high luminescence efficiency and because of their high brightness and ease of fabrication by solution processing, such as by spin casting and lithographic printing. On the other hand, it is still desired to find materials having pure emission spectra and good stability to produce cost effective OLEDs having lower driving voltages and higher efficiencies.
It is an object of the invention to provide an organic material based on [5] helicene or dibenzofluorene suitable for EL devices, having good thermal and morphological stability. In preferred embodiments, these materials are provided with hole transport or electron transport capabilities, in addition to emissive characteristics. These materials are expected to have improved charge injection mobility and charge recombination properties and pure emission spectra.
In preferred embodiments, aromatic amines are bonded to the [5] helicene or dibenzofluorene structures to provide hole transport capability to the subject compounds. Alternatively, electron transport moieties can be bonded to the [5] helicene or dibenzofluorene structures to provide electron transport capability to the materials. Electron deficient species, such as (without limitation) oxadiazole, thiadiazole, and triazole units, may be used for this purpose.
The invention may be embodied as an electroluminescent device, comprising a transparent anode, a cathode, and a layer of emissive [5] helicene or dibenzofluorene material between the anode and the cathode. Electroluminescent devices according to the invention may include an optional hole injection layer adjacent the anode or an electron transport layer adjacent the cathode. In another aspect, the invention encompasses an EL device incorporating a [5] helicene compound or a dibenzofluorene compound having both light emissive and hole transport ability, or both light emissive and electron transport ability.
The invention also encompasses a method of making emissive, hole transport organic materials based on [5] helicenes or dibenzofluorenes by aminating a dibromo [5] helicene or dibenzofluorene in the presence of a phosphorous ligand and Pd(0) catalyst in basic conditions. Similarly, emissive electron transport materials based on [5] helicenes or dibenzofluorenes can be synthesized by attaching an electron withdrawing group, also by means of a Pd(0) catalyst.
This brief summary has been provided so that the nature of the invention may be understood quickly. A more complete understanding of the invention can be obtained by reference to the following detailed description of the preferred embodiment thereof in connection with the attached drawings.